This invention relates to the field of radar target simulating, radar operating verification, and the calibration of radar transmitting and receiving apparatus. The invention particularly relates to passive and signal activated permanent echo target devices.
Radar target simulator devices of both the active and passive type have been known since the use of radar equipment during World War II. Passive signal return devices, that is, devices which return a received signal without amplification or new signal generation have been frequently used with radar sets for both friendly and hostile purposes; an example of such signal return devices is found in target simulating chaff--pieces of metallic foil cut to radar wavelength and dropped from moving aircraft. Other more complex signal return devices are also used with both airborne and ground based radars.
Current practice in the ground based search radar art is to locate unmanned or minimally attended radar equipment at remote sites which are coupled by low frequency telephone quality communication lines to a fusion center where the targets detected by a plurality of radar sets are correlated and merged into a single composite display. The limited communications capability between each radar site and the fusion center makes it desirable to remove unnecessary information from the output of each radar set before transmitting the remaining information to the fusion center. Although the moving target indication (MTI) techniques, which are known in the art, help in performing this removal process, in many instances, strong ground clutter signals and other undesired signals override the moving target indication circuitry and would be transmitted to the fusion center except for the use of target velocity suppression threshold circuits in each radar set. This velocity threshold circuitry is usually adjusted to remove from the radar output signal targets having a velocity below some predetermined minimum such as 50 miles per hour.
The use of remote radar locations and limited capacity communications lines therefore requires target tracking with the suppression of apparent velocities less than some threshold valve, in addition to MTI. Signals presented by a target simulator for use with such radars must therefore appear both to move in range and exhibit apparent doppler shift in order that information reaching the fusion center pass both the MTI and velocity "filters" and be usable for a complete evaluation of the remote radar equipment. It is also desirable for complete evaluation purposes that the information reaching the fusion center be indicative of the minimum discernable signal (MDS) capability of the remote radar set. For this MDS capability, the presence of amplitude modulation or target cross section variation capability is also desirable in a target simulator apparatus used with these radars.
A variety of radar signal return devices or target simulators is also described in the prior art as is shown by the following examples from the U.S. patent art.
The patent of Kenneth A. J. Warren, U.S. Pat. No. 4,319,247, discloses a radar test apparatus which receives a sample signal from a radar transmitter by way of a sampling antenna located close to the radar antenna. The Warrent apparatus processes this sample signal with a delay line and frequency shifter operating at radio frequencies before retransmitting the signal back to the radar by way of the same sampling antenna. The Warren apparatus comtemplates returning a plurality of pulses back to the radar set, each return pulse being of successively different amplitude in order that radar receiver sensitivity be evaluated. In the Warrent patent the apparent location of the returning signals is determined by the properties of a signal delay line rather than by atmospheric propagation time.
The patent of Willis S. Heyser et al, U.S. Pat. No. 3,103,010, discloses a radar test signal generating apparatus wherein signals received by a horn or antenna are offset in frequency through the use of a grid controlled klystron amplifier tube, and reradiated by the same antenna. The frequency or phase relationship of the received and transmitted signals in the Heyser apparatus is altered under the control of a sawtooth voltage generator which is preferably arranged to change the phase of the microwave signal by exactly 1 radio frequency cycle. The Heyser patent contemplates maintaining constant test signal amplitude and varying only phase or frequency in the returned signal in order to simulate the appearance of a moving radar target.
The patent of Edwin C. Adams, U.S. Pat. No. 3,329,953, discloses a radar target simulator which is intended for use with a doppler radar. The Adams patent teaches the use of a radio frequency circulator apparatus which returns the received radio signal without mixing or modulating and without the creation of sideband frequencies. The Adams patent also discloses the use of a bidirectional solidstate phase shifter which is controlled by doppler signature driver circuit to vary the phase of the returned frequency signal.
The U.S. Patent of John P. Chisholm, U.S. Pat. No. 3,108,275, is described in the above Adams patent and discloses an aircraft apparatus for returning a radio frequency signal to a radar set. The Chisholm patent combines an antenna and a semiconductor diode modulator; the diode receives a radio frequency modulation signal that is selected to provide a nonscintilating (i.e. fixed source location) radar return signal in both a sky tracking and beacon tracking mode of radar operation. The antenna of the Chisholm patent can be either a microwave horn or a dipole structure.
The patent of Donald J. Sommers, U.S. Pat. No. 3,331,070, discloses a moving target simulator apparatus. This apparatus uses a microwave diode that is driven by a multivibrator circuit to vary the propagation path lengths of the signal returned to the radar set by one-half wavelength or 180.degree. of radio frequency phase.
The patent of Norbert E. Tackman, U.S. Pat. No. 3,750,173, discloses a frequency translating repeater for returning radio frequency energy to a radar set in frequency shifted form. The Tackman apparatus contemplates the use of two semiconductor diodes which are biased in phase quadrature by an alternating current source to generate sideband frequencies that are a mixture of the radio frequency and modulator frequencies. The Tackman apparatus contemplates the return of signals at the original frequency and at one sideband frequency.